authorityKeyIdentifier and subjectkeyIdentifier inplement in the real case or model

[u8142010 <u8142010@xxxxxxxxxxxxxx>]

Hi:
I read the X.509 V3 document and felt some confusing with
authorityKeyIdentifier and subjectKeyIdentifier.
Does authorityKeyIdentifier and subjectkeyIdentifier inplement in the
real case or model, and how to implement the flow?
If without subjectkeyIdentifier, will verification of client's certs be
confused or fail? And in what situation it happens?

Best Regards,
James L.